1. Field of the Invention
The field of the present invention relates to an improved automated method for chemically milling metal and metallic structures. Chemical milling is widely employed in the aircraft and aerospace industries to remove excess metal from metal parts wherein the removed metal is not essential to the strength of the component part. The chemical milling process normally employs a series of masking and metal removal steps. The metal is removed by an etching bath which may be either caustic or acid depending upon the metal or alloy being etched. Chemical milling may be used to produce one piece structures having a skin and load bearing ribs or stiffeners that provide lightweight alternatives for traditional aircraft skin and stringer constructions.
2. Discussion of the Prior Art
The prior art has used chemical milling to reduce the weight of metal parts intended for use in aircraft or aeronautic applications for over twenty years. Chemical milling is widely used to increase the strength to weight ratio of component parts in the aircraft airframe. Chemical milling traditionally involves the steps of masking and chemically milling a metallic work piece and may repeat the sequence several times to further alter the work piece configuration.
U.S. Pat. No. 4,137,118 discloses a method of chemically etching an efficient light weight structure by removing excess metal to form the ribs and skin of an aircraft structure. The etching step is repeated to sequentially undercut and impart an "I" or "T" section to the ribs and to reduce the thickness of the skin.
U.S. Pat. No. 3,745,079 discloses a method of chemically etching a titanium alloy stock for use as a structural member in an aircraft.
U.S. Pat. No. 2,888,335 discloses a process of chemically etching a work piece that sets forth a method of sequentially etching the work piece with multiple cuts in the mask material to produce a plurality of etched levels in the work piece. In between each etching bath, a portion of the mask is removed so that the final configuration has an etch pattern of varying depths throughout the work piece.
U.S. Pat. No. 3,380,863 discloses masking material for use in chemical etching having a styrene/butadiene block copolymer composition. This material is widely used in chemical milling processes for masking the part to be protected during the etching bath.
At the present time, the prior art methods comprise the steps of marking the aluminum or titanium stock with a reference mark or tooling hole for conveying the stock through the etching solution. The metal part is then covered or coated with the butadiene/styrene copolymer masking material. A template is laid over the masking material and the masked material is hand-cut along the template line. The masking material is then marked with a marking pen along the cut or scribe line. These steps are repeated with separate masks when separate etching or milling levels are contemplated. In the event multiple etching baths are desired, the cut mask line is recovered with a sealant material. After all of the stencil marks have been cut, and the secondary cuts have been coated, one portion of the mask material is removed. The metal plate is then etched and rinsed in a counterflow rinse water. The second mask area is removed, and the work piece is reimersed in the etching bath. The work piece is rinsed again and the process is repeated for the desired number of etching steps. At the conclusion of the etching step, the work piece is "desmutting". A typical "desmutting" agent is disclosed in U.S. Pat. No. 3,988,254.
There are two problems in the present prior art method of chemical milling that are solved by the present invention.
(a) controlling the depth of the cut through the masking material. At present, the mask is handscribed by skilled workmen. If the cut is too deep, the cut allows the etching bath to etch into the metal and undercut the mask. If the cut is too shallow, and the mask material is not completely severed, it will "blowout", blister, or tear when that portion of the mask is removed. This necessitates a time-consuming repair step for the stencil mask. In addition, if the "blowout" is not detected, the work piece will be undercut by the etching material.
(b) the time consumed in laying each stencil on the work piece, marking each line to be cut, and cutting each line by hand is substantial. A typical three foot by four foot work piece requires six to eight hours of hand labor to handmark and cut each of the areas to be etched. The automated process of the present invention can do the same larking and cutting in 11 minutes. In addition, it can perform cuts that cannot be done by hand.
If the chemical milling is done on a three dimensional work piece, all of the foregoing problems are accentuated. In addition, it is necessary to preform the metal part around a master mold in a molding or die-stamping step to provide the desired three dimensional configuration. Each of the stencils must be provided with the appropriate compensation for three dimensional positioning. In the present prior art practice, after the metal plates have been preformed to an approximate three dimensional configuration, they are pinned to a master mold, and the individual stencils are also pinned to provide intimate contact between the stencil and the work piece. Further, the three dimensional nature of the work piece makes it even more difficult to accurately hand-cut the stencil to the desired depth.
The present invention involves the new use of two existing devices which have heretofore been used for other tasks.
In the drafting and cartography fields, large computer operated drafting machines have been used to mark blueprints and scribe plastic stencils that are intended for use in photoreproduction processes. These machines are quite large, having a drafting area that may be 8 feet wide 34 feet long. A motorized carriage traverses the drafting bed in both the x and y dimensions and carries on its carriage a plurality of marking pens. One such device is the Kongsberg 1800S Series Flatbed Drafting Table. This drafting table may be fitted with a variety of drafting tools including a tangentially controlled scribing tool. This tool uses a single knife or chisel and is normally used for cutting and stripping material used in the photoreproduction of integrated circuits. The knives are used to scribe coated films.
U.S. Pat. No. 3,555,950 discloses a device for automatically cutting a photomask for use in producing integrated circuits. In this device, the aluminum foil is cut and the plastic laminent material is retained to define an optically transparent negative for producing an integrated circuit board.
Computer controlled cutting means have been widely used in the garment industry for cutting one or more sheets of fabric to a desired pattern size. These devices also have drive motors for moving a cutting tool in x and y directions. Examples of computer operated cutting devices are disclosed in U.S. Pat. Nos. 3,803,960, 3,805,650, 3,895,358, 3,991,636 and 4,171,657.
While the foregoing devices have been used for computer controlled cutting of cloth and photomasks in the prior art, they have not been used or applied to the chemical milling process. The chemical milling process has remained essentially unchanged for over 20 years. The computer controlled flatbed drafting tables and cutting tables have been in existence for over 10 years. To the best of applicants' knowledge, these devices have not been used in the chemical milling process and their use in this field provides significant advantages in both speed and accuracy.
The foregoing devices, while suitable for application to chemical milling involving flat stock, are not suitable for use in chemical milling processes on three dimensional work pieces. For three dimensional milling, the tangentially controlled scribing tool is mounted on a robotic device that may be computer controlled through the x,y,z dimensions to provide an accurate scribing depth as the robotic device traverses the three dimensional contoured surface. One robotic device that may be modified for use in the chemical milling process is manufactured by ASEA, Inc. and is described in the ASEA pamphlet YB 11-101 E.
Both the ASEA robotic device, and the Kongsberg Drafting Table are capable of traversing an existing template to derive a series of point-by-point measurements along the perimeter defined by the template. These point-by-point measurements may be recorded and stored on magnetic tape. These point-by-point measurements may then be used to scribe the mask covered work piece with the tangentially controlled scribing tool carried by the Kongsberg plotter or the ASEA robot.
An alternate means of generating the instructions for controlling the movements of the robotic device or the flatbed drafting device is to create new template geometry on a CRT via an existing computer program that is currently sold under the "CADAM" tradename. This program will define the x and y coordinate values of the newly created mask before they are digitized and stored on magnetic tape.